SQL Support in Qt

The Qt SQL module uses driver plugins to communicate with several database APIs. Qt has drivers for SQLite, MySQL, DB2, Borland InterBase, Oracle, ODBC, and PostgreSQL. It is also possible to develop your own driver if Qt does not provide the driver needed.

With the MySQL driver, it is possible to connect to a MySQL server. In order to build the QMYSQL Plugin for Unix or macOS, you need the MySQL header files as well as the shared library, libmysqlclient.so. To compile the plugin for Windows, install MySQL.

If you use the embedded MySQL Server, you do not need a MySQL server in order to use that database system. In order to do so, you need to link the Qt plugin to libmysqld instead of libmysqlclient.

The Qt SQLite plugin is very suitable for local storage. SQLite is a relational database management system contained in a small (~350 KiB) C library. In contrast to other database management systems, SQLite is not a separate process that is accessed from the client application, but an integral part of it. SQLite operates on a single file, which must be set as the database name when opening a connection. If the file does not exist, SQLite will try to create it.

SQLite has some restrictions regarding multiple users and multiple transactions. If you are reading or writing on a file from different transactions, your application might freeze until one transaction commits or rolls back.

XML Support in Qt

Qt provides APIs to read and parse XML streams, and also to write to these streams. The following key classes facilitate these actions by providing the necessary infrastructure:

QXmlStreamReader class provides a parser to read XML. It is a well-formed XML 1.0 parser that does not include external parsed entities.

It understands and resolves XML namespaces. For example, in case of a StartElement, namespaceUri() returns the namespace the element is in, and name() returns the element's local name. The combination of namespaceUri() and name() uniquely identifies an element.

It is not CPU-intensive, as it doesn't store the entire XML document tree in memory. It only stores the current token at the time it is reported.

It is a simple API that provides a dedicated function for every XML token or event you want to write.

It takes care of prefixing namespaces based on the namespaceUri specified while writing elements or attributes. If you have to use certain standardized prefixes, you can force the writer to use them by declaring the namespaces manually with either writeNamespace() or writeDefaultNamespace().

It can automatically format the generated XML data by adding line-breaks and indentation, making it readable. This feature can be turned on with the auto-formatting property.

It encodes XML in UTF-8 by default. Different encodings can be enforced using setCodec().

Besides reading and writing to XML streams, Qt also provides APIs for the following additional use cases:

JSON in Qt

JSON is a text-based open standard for data interchange that is easy to read and parse. It is used for representing simple data structures and associative arrays, called objects. It is related to JavaScript, but is a language-independent notation form.

Local Storage

The Local Storage API provides the ability to access local offline storage in an SQL database from QML and JavaScript.

These databases are user-specific and QML-specific, but accessible to all QML applications. They are stored in the Databases subdirectory of QDeclarativeEngine::offlineStoragePath() as SQLite databases (SQL Database Drivers).

The API conforms to the Synchronous API of the HTML5 Web Database API, W3C Working Draft 29 October 2009 (HTML5 Web Database API).

QSettings Class

The QSettings class provides persistent storage of application settings. An application usually remembers its settings from the previous session.

Settings are stored differently on different platforms. For example, on Windows they are stored in the registry, whereas on macOS they are stored in XML files.

QSettings enable you to save and restore application settings in a portable manner. Constructing and destroying a QSettings object is lightweight and fast. While creating an object of QSettings, it is a good practice to specify not only the name of the application, but also the name of your organization. For example:

Resources

The Qt Resource System is a platform-independent mechanism for storing binary files in the application's executable. This is handy if your application frequently needs a certain file, or set of files. It also protects against loss of that particular file .

Resource data can either be compiled into the binary and accessed immediately in the application code, or a binary resource can be created dynamically and registered with the resource system by the application.

By default, resources are accessible from the application code by the same file name as they are stored in the source tree, with a :/ prefix, or by a URL with a qrc scheme.

File Archiving

An archive file is a collection of files or directories which are generally compressed in order to reduce the space they would otherwise consume on a drive. Examples of archive files are ZIP, TAR, RAR and 7z.